Various systems, such as a wireless communication system including mobile phones, are sometimes required to test whether they work properly. A signal analyzer may be used for measuring signals of the system under test. The signal analyzer can analyze a signal from viewpoints of both time and frequency domains. It converts the signal under test into digital data of the time domain and generates digital data of the frequency domain from the time domain data by FFT (Fast Fourier Transform) calculation. The resultant data are displayed as a waveform and/or numeric values on the display of the signal analyzer. The signal analyzer shows the time domain data corresponding to the frequency domain data of the signal under test, which makes it possible to provide various analyses from viewpoints of both the time and frequency domains. U.S. Pat. No. 6,377,617 discloses how to produce the frequency domain data with keeping correspondence relationship with the time domain data.
FIG. 3 is a conventional view of the frequency domain data on the signal analyzer 10. A graph area 40 displays the frequency domain data as a spectrum graph of which horizontal axis is frequency and vertical axis is power (dBm). A graph area 42 displays the frequency domain data as a spectrogram graph of which horizontal axis is frequency, vertical axis is time in frame number and level of the power is shown in color. A new frame is displayed in order from the bottom. The colors may be subject to the order of rainbow colors wherein if the power is higher the color is closer to red and the power is lower the color is closer to violet.
A square marker 46 on the graph 42 is used for designating a desired frame 48 by dragging the marker with a mouse cursor 44 by the user. The frequency domain data of the frame 48 designated by the center point of the marker 46 is displayed as a spectrum on the graph area 40.